These machines usually comprise a longitudinal bench over which a belt for moving the slabs to be smoothed or polished travels, two bridge support structures arranged astride the belt, one on the entry side for the material to be machined and the other one of the exit side for the machined material.
A mandrel carrying beam is supported at its opposite ends by two bridge structures. The beam is fitted with a series of vertical-axis smoothing and/or polishing mandrels which are arranged in a row and have, mounted on their bottom ends, supports which rotate about the vertical axis of the mandrel and are in turn fitted with abrasive tools, as will be explained more clearly below.
The beam may be fixed in position, if the working area of the smoothing or polishing mandrels is able to cover the entire width of the slabs to be smoothed/polished.
As occurs most frequently, however, since the slabs to be machined are very wide, the beam is slidably supported on the two bridge structures so as to perform an alternating rectilinear movement transverse to the direction of feeding of the material so that the working area of the tool carrying mandrels is able to cover the entire width of the slabs. The degree of translation varies depending on the width of the material being machined.
The tools used are made of hard granular materials, for example normally silicon carbide or diamond. In industrial applications the abrasive granules usually are not used in loose form, but are bonded together to form an abrasive tool by means of a bonding matrix (which may be a cement, a resin, a ceramic or a metal), this having the function of retaining the granules for as long as they are able to be perform their abrasive action, before eventually flaking and releasing the granules once worn.
The abrasive tools, as mentioned above, are normally fixed to a support which is rotationally driven by a vertical-axis mandrel.
In the case of soft stone materials, such as marble, the support for the tools, which have a prismatic form with flat surfaces, is generally an abrasive carrying plate.
In the case of hard stone materials, instead, such as granite or quartz, the support is usually a head which imparts a specific movement to the tools which are differently shaped and in any case in a spoke-like arrangement. The head may be of the type with oscillating supports (so-called oscillating-segment head) or rotating supports which have a substantially horizontal axis for roller-shaped tools (so-called roller head) or rotating supports with a substantially vertical axis for flat tools (so-called flat-disc head or also planetary or orbital head).
The tools also have a grain size which gradually decreases (from a few hundreds of micrometers to a few micrometers) as the slab passes underneath them. In particular, the first mandrel which machines the slab to be smoothed has tools with a relatively large grain size, the second mandrel has tools with a grain size which is slightly smaller and so on, while the tools with a very fine abrasive grain are mounted on the last mandrel.
The mandrel is slidable vertically and imparts to the tools resting on the surface of the material a pressure which may be of a mechanical, hydraulic or pneumatic nature; pneumatic pressure is by far preferred and this case the mandrel, or so-called “plunger”, is slidable vertically, being operated by a pneumatic pressure.
In this way a slab with a good polished finish is obtained. A number of drawbacks, however, exist.
In fact the mandrel carrying beam, and the mandrels associated with it, performs an alternating rectilinear movement transversely to the direction of feeding of the material and therefore, when the movement is reversed, there is a momentary pause in the movement of the mandrels and therefore of the smoothing or polishing tools. This pause produces a very slight local depression in the material which is sufficient, however, to create shaded zones above all on the polished surface of particularly delicate dark materials.
In an attempt to attenuate this effect, other machines have been devised where the mandrels are installed on rotating cross-like supports so as to cover the entire width of the slab to be machined. In particular, the mandrels can be positioned along the arms of the cross-pieces so as to be able to smooth and polish slabs of variable width.
Although this solution has been devised precisely in order to overcome the abovementioned problem of shaded zones, it has been noted, however, that in these machines the mandrels mounted on the cross-piece perform repetitive strokes with a coverage, or more specifically, stay time in the various zones of the material which is not uniform. This fact results in the creation, on the surface of the material, of bands with a varied polishing effect visible also to the naked eye.